Process for the utilization of ferrous sulfate solutions in the manufacture of ferric alums



Aug. 31, 194s. C. B. FRANCIS 2,448,425

f PROCESS FOR THE U'TILIZATION OF FERROUS SULFATE SOLUTIONS IN THEMANUFACTURE OF FEHRIC ALUMS Filed Jan. 50, 194'? l 2 Sheets-Shea?I l 5d0770/1/ $545.5 l:- I E; 1 @QZ/6 ug- 31, 1.948- c. B. FRANCIS 2,443,425

PROCESS FOR THE UTILIZATION OF FERROUS SULFATE SOLUTIONS IN THEMANUFACTURE OF FERRIC ALUMS Filed Jan. 30, 1947 2 Sheets-Sheet 2 M 9(Hmmm/25.

` (mm/m6@ j Olm/W5 MI y T @www 1L wwf/5mg f1 .I I?" /f//f @WMM/75m y nMw y (H4/@M5 Fm 05, @y @mau/1% Patented Aug. 31,v 1948 PROCESS FOR THEUTILIZATION OF FER- ROUS SULFATE SOLUTIONS IN THE MAN- UFACTURE OFFERRIC ALUMS Charles B. Francis, Pittsburgh, Pa.

Application January 30, 1947, Serial No. 725,402

This invention is a division of my copending application, Serial No.482,460, filed April 9, 1943, now Patent No. 2,416,744, and relates tothe manufacture of dibasic sulphates, such as ferric ammonium sulphate,from ferrous sulphate. More particularly, it relates to a process forutilizing spent pickling acid and crude gas or coke works ammonia toproduce either commercially pure ferrie ammonium sulphate commonly knownas ferric ammonium a-lum or certain other ferrie alulns such` as ferriesodium alum, pure ammonia and ferrie hydroxide.

Spent pickling acid is produced in the cleaning, or pickling, withsulphuric acid of steel articles, such as sheets and wire, that are tobe cold formed or coated with another metal, and represents a crudesolution, heretofore considered as a waste product from steel plants,containing 2% to 7% free sulphuric acid and `10% to 30% ferrous sulphateas principal ingredients and, in addition, smaller proportions ofinsoluble substances consisting of organic matter, traces of ferricsulphate and the salts of various metals, such as manganese, that werealloyed With the steels pickled.

Likewise, crude aqua ammonia as obtained from the ammonia stills ofby-product coke and gasworks represents a complex solution of variousammonium compounds as the principal constituents, mainly ammoniumhydroxide and ammonium carbonate with smaller percentages of ammoniurnsulphides and cyanides and numerous organic compounds in still smallerproportions.

The crude aqua ammonia I prefer in the operation of my process containsfrom to 25% total ammonia, expressed as per cent NH3 by weight, and isknown as concentrated liquor, obtained by treating weak liquor withcalcium hydroxide in ammonia stills. The concentrated liquor `producedby coke works usually contains about 18% total ammonia.

Because the iron in the spent pickling acid is in the ferrous conditionand the crude aqua arnmonia contains acid radicals which form solublecompounds with ferrous iron, al1 of the latter cannot be precipitatedand separated by adding ammonia to the spent acid directly, nor canother bases be used to .accomplish this object. If the ferrous sulphatein the spent acid is oxidized to ferric sulphate, however, all the ironis readily precipitated by ammonia and several other bases, givingferric hydroxide, which is insoluble and can be separated from thesolution of sulphate containing the metal substituted for iron. Of themany known ways of oxidizing Vferrous sul- 9 Claims. (Cl. 23-118) phatein solution, all of them are too costly to be commercially applicable tothe treatment of spent pickling acid for the production of othersulphates. However, in studying these methods, I have found that theknown process of oxidizing ferrous sulphatevlith sulphur dioxide and airmay bemodied to make it practicable for the commercial utilization ofspent pickling acid and crude concentratedcoke works ammonia in themanufacture of other sulphates, particularly ferric ammonium sulphate,which may be used to produce other sulphates, pure ammonia and a ferric`hydroxide that may be applied to many purposes. Other bases which maybe substituted for ammonia in the treatment of the spent acid to producesulphates include the hydroxides or oxides of llithium, potassium,sodium, magnesium, uranium, aluminum, manganese, zinc, cadmium, cobalt,and nickel.

However, in thefollowing description of my process, I shall employ crudecokeworks ammonia as an example of the bases Which may be used in theinitial (neutralizing and oxidizing) steps of my process, although theutilization of spent pickling acid for the manufacture of ferricammonium sulphate has been one of the chief objects oi my invention.

i In experimenting with this method of oxidizing ferrous sulphatesolutions a fourth discovery I have made is that sulphites, such asammonium sullphite or ammonium bisulphite, may be substituted for thesulphur dioxide, if added to the ferrous sulphate solution gradually ata rate that will not lower the hydrogen ion concentration of thesolution below pI-l 5. The sulphites are added, most conveniently in theform of concentrated solutions, the only requirement being that thesulphite used must contain the base of the sulphate desired.

Thus, with a supply of a given base and by slight modification in myprocess and some omissions or additions of equipment, I may start withacid or neutral solutions of ferrous sulphate, oxidize the ferroussulphate to ferric ,sulphate by the use of sulphur dioxide or sulphitesand air to produce either ferrie alum or a sulphate of the given baseand ferric hydroxide. As the use of spent pickling acid and crude coke-Works ammonia to produce ferric ammonium sulphate is the chief object ofthe invention, I shall rst describe in detail how I accomplish thisobject by the use of sulphur dioxide and air to oxidize the ferroussulphate.

By the known process of oxidizing ferrous sulphate in solution withsulphur dioxide and air,

the two reactions following take place simultaneously as soon as acertain proportion of the ferrous sulphate has been oxidized to ferriesulphate, the latter apparently acting as a catalyst to both reactions:

In the presence of mu-ch free sulphuric acid, say 4% or more, or in thepresence of small percentages of manganese sulphate, which also acts asa catalyst in reaction 2, reaction 1 progresses very slowly, so thatthis method for oxidizing iron cannot be commercially applied directlyto the spent pickling acid for oxidation of the ferrous sulphate itcontains. I have found, however, thatif the acidity of the spent acidsolution is adjusted to between approximately pH 2 and pI-I 4,

pI-I 2 to prevent the precipitation of ferric iron and pI-I 4 to preventthe precipitation of ferrous iron (hereinafter described as neutral)with ammonia or certain ammonium compounds, such as ammonium carbonate,ammonium sulphide or sulphite, that react with sulphuric acid to give agaseous product, reaction 1 progresses to the complete oxidation of theiron. I have further discovered that the crude aqua ammonia obtained inthe coking of coal and the manufacture of coal gases, or the vapors asthey come from the arnmonia stills regularly employed in the by-productcoke industry, may be used for the purpose of neutralizing the free acidand precipitating the iron and other metals in the spent acid that maybe separated by hydrolysis. Again, in applying the known method to theoxidation of ferrous sulphate in spent pickling acids, reaction 1progresses only to the oxidation of about half the iron in relativelyconcentrated solutions containing or more ferrous sulphate, whenreaction 2 begins, forming free acid which may again retard or haltentirely the progress of reaction 1, and this defect I have found isalso overcome by adding ammonia in proportion to the acid formed.

Again, as the proportionof ferrie sulphate increases and reaction 2 iscatalyzed, there is danger, particularly if the sulphur dioxide (SO2) isfed into the solution too fast for the following reaction to occur:

This reaction, in which the SO2 acts as a reducing agent, is well knownand frequently employed, particularly in analytical chemistry. I havefound that if ammonia is introduced under proper conditions into theoxidation cell, that is, the chamber in which the oxidation of theferrous sulphate is being carried out, the acid formed by reaction 2 maybe kept neutralized and reaction 3 prevented from occurring, except whenthe SO2 is fed at a rate out of proportion to the available oxygen asrequired by reactions 1 and 2.

All three of these rea-ctions are exothermic and progress most rapidlyat a temperature between 50 and 60 C. Consequently, the oxidation of theferrous sulphate is slow at the beginning, but eventually enough heat isgenerated in the solution to heat it to the optimum temperature and thereaction progresses most rapidly.

I havefalso found that ammonia may be used to accomplish another objectwith advantage to the process, explained as follows:

During the operation of an oxidation cell for the oxidation of a givenamount of a solution of ferrous sulphate, a certain small proportion,

.sulphate to form ammonium sulphate, ferrous sulphate and sulphuric acidaccording to reaction 4.

If free sulphuric acid is present, the ammonium sulphite reacts with itto form ammonium sulphate, which is a soluble salt, sulphur dioxidewhich is a gas, and water which is a non-electrolyte according to thefollowing reaction:

With an excess of diffused air fed to the oxidation cell, the ferroussulphate and sulphur dioxide formed by reactions 4 and 5 respectivelyare immediately oxidized in accordance with reactions 1 and 2. T'hesereactions are important in 'the operation orf the process, as they mustbe controlled to give the product desired.

Alfter as much as 99% or more of the ferrous sulphate in the spe-ntpick-ling aci-d has been oxidized as described above, the oxidizedsolution is preferably drawn :out of the oid-dation cell and transferredto another vessel Where it may be treated, in the event that there is anexcess of ferric sulphate present, with pure ammonia Ito precipitate theiron in accordance with the following reaction:

The excess iron is Kprecipitated to :adjust the proporti-on of ammoniumsulphate to ferrie sulphate to -f-orm ferric rammonium sulphate [(NH4)2SO4.Fe2 (S04) 3] The lcompleteness of the oxidation oaf Ithe `ferroussulphate depends largely upon the purity of the ammonia available. Withpure ammonia, I may prefer to -oxidize 100% of the iron, but lascomplete oxidation of the iron requires additional time out ofproportion to the iron oxidized earlier in the :process and crudeammonia contains substances that reduce ferric sulphate, I prefer toleave some unoxidized iron in the solution before adding crude ammoniarto precipitate the iron.

Crude ammonia contains, besides other impurities, sulphides and cyanideswhich reduce ferr-ic sulphates. The sulphides react Wirth ferrous andferric sulphates as follows:4

As is indicated lby reaction 7, ammonium sulphide in reacting withrfenri-c sulphate may, under certain circumstances, give free sulphur,Which Amay cause trouble by remaining in colloidal solution or byreacting with cyanide to form sulphocyanides (thiocyan-ates) which, inturn, react with ferric sulphate to form soluble .iron comcyanides reactwith the ferrous and ferrie sulphaltes to iorin insoluble lcyaan-des'und the rnee sulphur-formed in reaction 'l is less troublesome becauseat Ileast partof it may be enll'llpped by the -f-erric hydroxide andseparated with the latter upon iiltering. o

In my process the ammonium carbonate (NH4) 2G03 contained in the crudeaqua; ammonia serves a purpose similar to ammonium hydroxide, becausethe conditions are such that the carbonate is decomposed, formingammonium sulpirate and carbon dioxide, which, being a is carried out ofthe solution with the other gases or forced out by heat.

Having thus 'explained my discoveries relating to the steps ofneutralizing the `free acid with ammonia and of oxidizing the ferroussulphate Wirthy sulphur dioxide and air, I desire to describe how to`apply my next discovery, namely, ftJhalt `solutions of `sulphites,[particularly ammonium sulphite, may be substituted for sulphur dioxide.This ldiscovery is important, because the solutions of the sulphites aremore -elasily handled and controlled than the |gaseous, sulphur dioxide,land it Ialso permits .fthe utilization of sulphur dioxide from manysources, such as chimney gases, gases from the calcinamion of Wastes111- phur bearing material-s, etc., from which the sulphur ldioxide maybe recovered as sulphites by scrubbing the wlaste gases with Idiluteammonia or soda. solutions.

Using the ammonium sulphites as an example, Ithe principles of thismodiiieation of my invention are most brieily explained by the followingreamions:

With acid sulphate solutions the ammonium sulphites iirst react `withthe yfree acid Ipresenlt, thus:

Excess free acid results in the dormaltion of acid ammonium sulphate(las in reaction 12),

which, when :fully neutralized, will give am' monium sulphate in excessover that required to form cterric ammonium Ialu-m, thus:

To avoid precipitation of part of the iron as a hydrated ferrie oxide,neutral ferrous sulphate solutions require ammonium lbisulphilte in theoxidation step, this being represented by reaction 13:

From the preceding descriptions, it will beobserved that the oxidizedsolutions represent mixtures of ammonium` and'ferric 'sulphates and, if

the percent of free acid in the spent pickling acid is approximatelyone-third of the per cent of ferroussulphate,l as it generally is, thesesalts 'are` in proper proportion to `forln ferrie ammonium This doublesalt is very soluble in warm water-about 400 grams in 100 grams of waterat 100 C.-but much less soluble in cold water-only about 100 grams beingsoluble in V100 grams of water at 20 C. Consequently, if about one-halfof the water originally present in the spent picklng acid'is evaporated,the double salt will crystallize at room temperature. If the solution iscooled to some temperature below 10v C.; from one-half to two-thirds ofthe double salt will crystallize without evaporative treatment. Withneutral ferrous sulphate solutions oxidized with sulphur dioxide and airor bisulphltes and'air,`

the evaporation of the excess Water may be effected prior to oxidizing;but where sulphitesV are used the evaporation is done by preheatng thelair that is diffused into the solution in the oxidizing cell, or'preferably in a separate operation in which the solution is sprayedthrough preheated air contained in a Wooden or a lead lined steelvessel. In either case the solution is transferredto a crystallizingtank in which it is permitted to cool and from which the crystals are byconventional means.

This double salt may be used to advantage to prepare ferric sodiumsulphate as a substitute for alum in water treating as well as for manyother purposes. To prepare ferrie sodium sulphatet'he ferrie ammoniumsulphate is dissolved in 1A, to V3 its weight of boiling water and tothe hot solution is added an amount of sodium hydroxide equivalent tothe ammonia present, and the mixture is heated to boiling to liberateammonla and form ferr-ic sodium alum according to the followingreaction:

It is to be noted that the ammoniathus obtained is commercially pure,being contaminated only by the water vapor that distills, with which itcombines to form ammonium hydroxide. In the purication 'of crude cokeworks ammonia,` it is treated with calcium and sodium hydroxides andredistilled, but no lay-products of value are recovered. Therefore. thepresent invention represents a novel process for the production ofpureammonia and ammonium hydroxide from crude coke works ammonia.

If preferred, the oxidized solution may be used for the production ofammonium sulphate and ferrie hydroxide or oxide. If the latter productis desired, the oxidized liquor-'is treated as already described for theproduction of ammonium sulphate.

,The reactions by which I produce ferrie ammonium alum in at least fivemodifications of the processol.' my invention are as follows:

Alum ,from acid and neutral ferrous sulphate solutions oxidized withsulphur dioxide and air autres ferrous sulphate to nine partsA of freeacid tojjal.;vv

the oxidizing chamber until, as shown by analysis,

the sulphates are in correct proportions to form ferric alum.- -Irf theexcess water has previously been evaporated, this .point can bedetectedby a rapid analysis for ammonia which should bel 3.5 by weightof the solution. H

-Figure 1 is a flow sheet illustrating my inven- I; Figure 2 is a flowsheet similar to Figure 1, but in which sulphites are substituted forsulphur dioxide.v l `Referring to Figure 1 oi the drawings, thereferencenumeral I8 represents a pickling vat; I9

a combination neutralizing and filtering tank for spent pickling acid;19A a neutralizing and storage tank as is'used for otheracid ferroussulphate solutions; |9B a storage tank as is used for neutral ferroussulphate solutions; 20 a storage tank for asupply of the solution of thebase to be used, e. g., aqua ammonia; 2| evaporating equipment,preferably multiple eiect; 22 a-tank or chamber in whichthe ferroussulphate gsolutions are oxidized; 23 any source of supply of sulphurdioxide gas; 24 anaircompressor; 25 acooling and crystallizing tank ofconventional design; 26- a centrifugal dryer and 25A a storage andcrystallizing tank i supplementing 25, .particularly for the processingof spent pickling acid and similar substances that may be partially orwholly purified by crystallization. In such latter-solutions, the`impurities concentrate in thevk mother 1iquor, and this plant layoutprovides for running the mother liquor through the evaporatorsseparately if desired.

vFrom'the flow sheet, itis apparent .that the various steps in thismodification of my process are asffollows: t

\ The acid ferrous sulphates are just neutralized with the base of thenon-ferrous sulphatedesired and, if necessary, are freed ofinsolublematter. The neutral sulphate solutions are next evaporated toleave them -in solution in a quantity ofA water equal tothat requiredto-keep them in solutionat60iC. f

As the iron is most quickly determined, I use it as an index and aimto-obtain a certain numberbetween and lll-of grains 4of iron yper 100c.- c. of solution, the exact proportonsdepending, upon whetherv or notthere fare impurities .which it is desirable to separate by crystallization. In processing commercially pureferrous: sulphate lsolutionstoV -ferric ammonium alums, the, total. Water should be: equal totheweight of the salt including thewate'r of cry` stallization. Thesolution is then transferred to the oxidizing `chamber Where the ferroussul#` phate'is oxidized with sulphur'dioxide and air,

as previously described. The end point at "this" stage is determined bytitrating a small portion for ferrous iron, as in. a chemicaldetermination of iron.

vliter substantially .all of the iron. has been oxidized, the cell 22 isoperated to lproduce-acid as required, at the end of which period thehydrogen'fioii'concentration the solutionl is ad? justed to,V 15H2, andthe' solutlonis transferred to al crys'tallizing tank. Here'it'iscooledtoa 'teinperature between 10 and 20 C. and thevcrystals are separatedfrom the mother liquor in which Alum from acid and ueutralferroussulphate solutions, oxidized with sulphites jAumodification'of myprocessA for using ferrous vsulphate solutions for the manufacture offerrie alum in which sulphites are substituted for sulphur dioxide isshown in Figure 2, which is a flow sheet indicating the various stepsfor treating acidl ferrous sulphate and neutral ferrous"sul phates. ,Alsis apparent from this ow sheet, acid ,ferrous sulphate solutions areoxidized with sulphites and air while neutral ferrous sulphates areoxidized with bisulphites and air, each of these two treatmentsrequiring a Adifferent 'arrangement of equipment and in one instance,namely, evaporation, different types of equipment are required. T heprinciples of each modification are shown bythe followingv reactions:

For acid ferrous sulphates oxidized with sulphites, vfor example,iammonium sulphite and air, the reaction is- For neutral ferroussulphates oxidized with bisulphites, for example, ammonium bisulphiteand air, the reaction is-v Y 'These reactions give, for bothmodifications of the: process, the relations by Weight of the variousreagents required' to form the ferric alum desired and form the basisfor the control of the free acid, the quantitiesvof sulphites orbisulphites required and the percentage of water to be left in solutionprior to evaporation, which may be regulated as explained above foroxidationwith sulphur dioxide and air.

4vThe Various steps in the process, as Well as the ,modificationsrequired to treat acid and neutral i ferrous sulphate solutions areillustrated by Figure 2, which is a flow sheet giving the steps in theprocessing of acid ferrous sulphates (left) with sulphites and air, forexample, am-

monium sulphite and air,. and the processing of4 acid ferrous sulphatefrom any source, and 28B a storage tank for neutral ferrous sulphatefrom any source.

their Vrespective oxidation chambers, 30 and 30A,

as required.

From'reaction I6 it will be observed thatthe acid in the solution shouldbe` approximately one-.third of the ferrous Ysulphate by lweight,l butrI havefound that solutions containing less than; this yproportion ofacid may be used by continuingv this treatment beyond the point at Fromthese vtanks the acid solu. tions are conducted through acidproofpipesto" which all the iron is oxidized, the ammonium sulphite being oxidizedto sulphate when it is conducted into the solution after this point hasbeen passed. Therefore, I continue the treatment with diffused air,admitting the sulphite gradually until the iron is all oxidized andtests show that the iron and ammonia are in proper proportions to formferric ammonium alum, then stop the addition of sulphite and continuethe air iiow until a ltest indicates all the sulphite has been oxidized.The oxidized solution is then permitted to flow to a spray dryer 33 and33A in which the water in excess of that required for crystallization isevaporated with heated air or other hot gases of an oxidizing nature.From the evaporator, the saturated solution is conducted tocrystallizing tanks 34 and 34A, Where the liquor is cooled and the saltspermitted to crystallize. The crystals are then collected and dried in acentrifugal type dryer, while the mother liquor is returned to the dryeror collected in a special storage tank for treatment separately.

In treating neutral ferrous sulphate solutions, this procedure may bechanged and the excess water evaporated in a multiple effect evaporator33B before the solution is oxidized. After evaporation of the excesswater, the solution is conducted to an oxidizing cell 30B, similar inevery respect to 30 and 30A, used to oxidize acid ferrous sulphatesolutions. In this chamber, diffused air is admitted at the bottom andthe sulphite, for example, ammonium bisulphate, is fed in graduallyuntil all the iron is oxidized. This treatment automatically gives thesalts in proper proportion to form ferric ammonium alum, as shown byreaction 17. The solution is then transferred to a crystallizer andcooled, when the salts are recovered and dried by centrifuging asdescribed above for other modifications of the process.

For the purpose of oxidizing ferrous sulphate in solution, in accordancewith the present procless, the similarities as well as the differencesin the reactions obtained with SO2 and air, HzSOs and air, or sulphites,such as ammonium sulphites, and air, are brieiiy and simply explained asfollows:

First, it is noted that the oxidation of ferrous sulphate ZFeSOs toferric sulphate Fe2(SOu)3 amounts to changing the valence of iron from 2to 3 and that this change may be effected in two ways, namely, bycombining three molecules of ferrous sulphate and removing one atom ofiron from the group, as in reaction 18 below, or by combining twomolecules of the salt and adding one S04 radical as in reaction 19:

According to the prior art, reaction 19 above is represented by reaction19a below, which is also the reaction employed in the present process,and set forth in the specification to show the relation of the presentinvention to the prior art, it being understood that a disclosure is notnecessarily a scientic exposition.

Either of these reactions may be used to oxidize ferrous sulphate, butsince it is desired in the present invention to avoid the formation of aprecipitate as a product of the oxidation, I employ the second reaction19 above, substituting sulphites, specifically sodium 01' ammonium sul-10 phite or bisulphite, for the sulphurous acid. Incidentally, thesereactions express one of the fundamental differences between the presentprocess and the processes of the prior art.

Comparing the action of these sulphites with that of sulphurous acid, itis noted that all three of these ionize as follows:

From these ionization reactions, it is apparent that the active agent inthe oxidation of f-errous sulphate, in accordance with reactions 19 and19a is S03, which must first be oxidized to S04E by taking oxygen fromthe air used, a reaction which is catalyzed by ferric sulphate, manganicsulphate, vanadic sulphate, and possibly other sulphates of the heavymetals. It is, therefore, the free nascent S04 radical which is capableof uniting, and does so unite, with two molecules of ferrous sulphate toform ferric sulphate.

But, whereas the fundamental chemical principle underlying the oxidationof ferrous sulphate by sulphurous acid and by the sulphites, both actingin conjunction with atmospheric oxygen, it is the same, all three of theformer reagents present certain differences that are important in thepractice of the process of the present invention, which differences maybe listed briefly as follows:

l.. The oxidation of acid solutions of ferrous sulphate with sulphurousacid and air progresses slowly or not at all if the acidity is high, andin the oxidation of neutral solutions (pH 2 to pH 4) some sulphuric acidis formed as the ferrous sulphate is oxidized.

2. With ammonium sulphite, or bisulphite, highly acid solutions offerrous sulphate may be oxidized, because the sulphite first reacts withthe free acid, liberating sulphurous acid and forming ammonium sulphateby one of the following reactions:

In these reactions, the amount of free acid present determines whetheracid or neutral ammonium sulphate is formed.

3. With neutral ferrous sulphate solutions, that is, solutions of pI-I 4to pH 5, ammonium sulphite precipitates some iron as ferric hydrate,while ammonium bisulphate gives no precipitate. Although the ferrichydrate may be dissolved, I prefer to prevent its forming in order toavoid danger of forming an almost insoluble basic ferric sulphate. Theoxidation of acid and neutral ferrous sulphate solutions with ammoniumsulphite and bisulphite are represented by reactions 16 and 17.

li. After all the ferrous sulphate has been oxidized in a given batch ofsolution with ammonium sulphite, I may, in the operation of the process.continue the feeding of the latter and of diffused air to control theamount of ammonium sulphate formed in relation to the quantity of freeacid and ferrous sulphate originally present because the sulphite itselfis then oxidized to ammonium sulphate thus:

. The substitution of the ammonium sulphites for sulphur dioxide hasother outstanding advantages also. For example, the burning of pyritesor other sulphur-bearing compounds, and even sulphur itself, presentsdiiculties in that the mixtures of SO2 and air are very corrosive, thegases are laden with dust that clogs diifusers, and obviously all partsof a plant must operate in unison.

By the present process all of these difliculties are overcome, for theremust be absorbed the SO2 from the products of combustion of thesulphur-bearing compounds with dilute solutions of ammonia which may bebuilt up to any concentration desired, because the ammonium sulphitesare very soluble in water. These solutions may be prepared, stored, andused as required, independent =of the production and oxidation offerrous sulphate. Use of ammonia in this way to obtain the ammoniumsulphites also increases the sources of sulphur dioxide from which theoperator may draw to roperate the process, because he may thus useammonia to remove the sulphur dioxide from stack gases produced infurnaces burning coal or other high sulphur fuels. Dusts collected bythe sulphite solutions do not interfere seriously in the present processbecause they are separated easily from the solution and, if notseparated, they will not clog the apparatus employed, since applicantfeeds these solutions to his oxidizing chambers through tubes withrelatively large openings. Instead of ammonia, sodium hydroxide orcarbonate may be used to absorb sulphur dioxide for the directproduction of ferric sodium alum.

It may be mentioned in this connection that the present process opens upthe possibility of using ammonia for removing sulphur dioxide from stackgases, since the present process opens up an important source ofdisposal of the sulphite liquors, the disposal of which heretoforepresented a diiiicult problem.

It may be noted in this connection regarding the term base or bases asemployed in the following claims that a base, in general, is a substancewhich is capable of decreasing the hydrogen ion concentration ofsolutions as distinguished from acids which increase the hydrogen ionconcentration. Bases that form soluble sulphates and also are capable ofprecipitating ferric ions are the hydroxides and carbonates of ammonium,lithium, sodium, potassium, magnesium, zinc, and nickel, so that allthose citations of record that use calcium hydroxide or calciumcarbonate or other base forming insoluble sulphates, such as barium,cannot be regarded as anticipatory of the present invention because theychange the principles of the process and the nature of the productsobtained.

Having described in detail the various modifications that may be made inthe process of my invention, I desire to add that numerous minor changesmay be made in the procedure of carrying out the process withoutexceeding the scope of the invention as defined by the following claims.

I claim:

1. In the manufacture of commercially pure ferrie alums by reaction offerrous sulphate solutions and a suitable base, the process whichincludes the steps of adjusting the acidity of the solution to betweenapproximately pH2 and pli-I4 by the addition of the sa-id base,oxidizing the ferrous sulphate to ferric sulphate by treating thesolution at a temperature between 45 C. and 70 C. with diffused air anda compound of the class consisting of sulphites and sulphur dioxide,

continuing this operation withr the gradual addition of the base untilthe sulphate of this base and the ferrie sulphate are in properproportion to form alum, evaporating a major portion of any excess waterpresent, cooling the solution to effect crystallization and finallyseparating the crystals from the mother liquor.

2. In the manufacture of commercially pure ferric alums by reaction offerrous sulphate solutions and a suitable base, the process whichincludes the steps of adjusting the acidity of the solution to betweenapproximately pH2 and 11H4 by the addition of the said'base, oxidizingthe ferrous sulphate to ferric sulphate by treating the solution a1; atemperature between 45 C. and 70 C. with diffused a-ir and sulphurdioxide, continuing this operation with the gradual addition of the baseuntil the sulphate of this base and the ferrie sulphate are in properproportion to form alum, evaporating a major portion of any excess waterpresent, cooling the solution to effect crystallization andv finallyseparating the crystals from the mother liquor.

3. In the manufacture of commercially pure ferric alums by reaction offerrous sulphate solutions and asuitable base, the process whichincludes the steps of adjusting the -acidity of the solution to betweenapproximately pH2 and pH=l by the addition of the said base, oxidizingthe ferrous sulphate to'ferric sulphate by treating the solution at atemperature between 45 C. and 70 C. with diffused air and the sulphiteof the said base, continuing this operation with the gradual addition ofthe base until the sulphate of this base and the ferrie sulphate are inproper proportion to form alum, evaporating a major portion of anyexcess water present, cooling the solution to effect crystallization andfinally separating the crystals from the mother liquor.

4. In the manufacture of commercially pure ferrie alums by reaction offerrous sulphate solutions and a suitable base, the process whichincludes the steps of adjusting the acidity of the solution to betweenapproximately pH2 and pI-Ifi by the addition of the said base, oxidizingthe ferrous sulphate to ferric sulphate by treating the solution at atemperature between 45 C. and 70 C. with diffused air and a compound ofthe class consisting of sulphites and sulphur dioxide until alum isformed, crystallizing the alum in the solution and then separating thecrystals from the mother liquor.

5. In the manufacture of commercially pure ferric alums by reaction offerrous sulphate solutions and a suitable base, the process whichincludes the steps of adjusting the acidity of the solution to betweenapproximately pH 2 and pH 4 by the addition of the said base, oxidizingferrous sulphate to ferric sulphate by treating the solution at atemperature between 45 C. and '70 C. with diffused air and sulphurdioxide until alum is formed, crystallizing the alum in the solution andthen separating the crystals from the mother liquor.

6. In the manufacture of commercially pure ferric alums by reaction offerrous sulphate solutions and a suitable base, the process whichincludes the steps of adjusting the acidity of the solution to betweenapproximately pI-I 2 and pH 4 by the addition of the said base,oxidizing the ferrous sulphate to ferrie sulphate by treating thesolution at a temperature between 45 C. and 70 C. with diifused air andthe sulphite of the said base until alum is formed, crystallizing thealum in the solution and then separating the crystals from the motherliquor.

7. In the manufacture of commercially pure ferric alums, ammonia andammonium hydroxide by reaction of ferrous sulphate solutions, ammonia,and a suitable base, the process which includes the steps of adjustingthe acidity of the solution to between approximately pH 2 and pH 4 bythe addition of ammonia, oxidizing the ferrous sulphate to ferricsulphate by treating the solution at a temperature between 45 C. and 70C. with diiused air and a. compound of the class consisting of ammoniumsulphite and sulphur dioxide until ferrie ammonium sulphate is formed,then adding the said base to form alum, heating the resulting solutionto drive off the ammonia as a gas or vapor and then recovering the alumand the ammonia by conventional methods of condensing and compressing.

8. In the manufacture of commercially pure ferrie alums, ammonia andammonium hydroxide by reaction of ferrous sulphate solutions, ammonia,and a suitable base, the process which includes the steps of adjustingthe acidity of the solution to between approximately pH 2 and pH 4 bythe addition of ammonia, oxidizing ferrous sulphate to ferrie sulphateby treating the solution at a temperature between 45 C. and '70 C. withdiffused air and sulphur dioxide until ferrie ammonium sulphate isformed, then adding the said base to form alum, heating the resultingsolution to drive oi the ammonia as a gas or vapor and then recoveringthe alum and the ammonia by conventional methods of condensing andcompressing.

9. In the manufacture of commercially pure ferric alums, ammonia andammonium hydroxide by reaction of ferrous sulphate solutions, ammonia,and a suitable base, the process which includes the steps of adjustingthe acidity of the solution to between approximately pH 2 and pI-I 4 bythe addition of ammonia, oxidizing the ferrous sulphate to ferriesulphate by treating the solution at a temperature between 45 C. and 70C. with diffused air and ammonium sulphite until ferrie ammoniumsulphate is formed, then adding the said base to form alum, heating theresulting solution to drive off the ammonia as a gas or vapor and thenrecovering the alum and the ammonia by conventional methods ofcondensing and compressing.

CHARLES B. FRANCIS.

REFERENCES CITED The following references are of record in the file ofthis patent:

'UNTED STATES PATENTS Number

